Coherent beam combination (CBC) of laser amplifiers is a well-established technique for locking multiple laser emitters in phase with one another to form a high brightness beam. Typically, the output from a low-power master oscillator is split into a multiplicity of beams, each of which is passed through a laser amplifier to increase its power. The amplified output beams are combined geometrically and phase-locked to each other. The combined beam behaves as if it were emitted from a single aperture laser, but with higher brightness than can be obtained from an individual laser. The CBC imposes a requirement that the optical path length through each laser amplifier in the phase-locked array must be matched to within a small fraction of the master oscillator's coherence length. If the optical path mismatch between any two elements exceeds the coherence length, then the two elements will appear to be incoherent with one another, and they cannot be successfully phase-locked. Even if the optical path mismatch is only a fraction of the coherence length, the coherence between the two lasers will be less than 100%, leading to a reduction in the array brightness.
Laser beams in a coherently combined array must also be co-aligned with one another to achieve maximum combining efficiency. Each of the lasers must have their beam footprints and their pointing directions overlapped to within a small fraction of the diffraction limit. This is difficult to achieve with high power lasers due to assembly tolerances, dynamic changes in beam parameters, thermal expansion of mechanical fixtures due to stray light absorption, and platform deformations, for example. These problems are particularly acute for systems deployed outside a controlled laboratory environment. Hence, there is a need for active beam pointing and position control systems to maintain coherent combining efficiency.
With a large channel count array, active controls can be cumbersome due to the difficulty of sensing beam parameters for every laser. Most conventional systems require the use of arrays of sampling optics and quad-cell or other position-sensitive detectors (PSDs), one for each input laser, to diagnose misalignments. Large parts count and opto-mechanical complexity make this approach unattractive for deployable systems outside the laboratory. Another disadvantage of multi-detector sensing is that misalignments can be quite subtle and difficult to detect. For example, ±1-μm tip displacements can lead to 1% combining loss for a 20-μm core fiber. At this required high level of precision, slight changes in the relative position or responses of different detectors can easily be misinterpreted as changes in the laser array, thus reducing control fidelity.
For these reasons, single-detector methods of sensing array misalignments provide an attractive alternative to detector arrays. Conventional implementations of single-detector position sensors have required dithering (e.g., causing small beam misalignments) the beam pointing or position, and sensing the resulting loss of combining efficiency. This is undesirable since dithering unavoidably reduces the control precision and limits the final combining efficiency. Moreover, this method does not scale adequately to large arrays (N beams ˜>100) since the error signals become attenuated and control bandwidth drops as 1/N.